1. Field of the Invention
This invention relates to a control system for a selective oxidizer.
2. Description of the Prior Art
A selective oxidizer is a well-known device used to reduce the level of carbon monoxide in a gas stream. For example, it is often used in combination with fuel cells to clean the reactant gas fed to the cells since carbon monoxide poisons the cells by reducing the activity of the cell catalyst.
A typical fuel cell uses phosphoric acid as the electrolyte and operates on air as the oxidant and hydrogen as the fuel. To produce economical power on a large scale, fuel cell power plants must utilize inexpensive hydrocarbon fuels. These fuels must be converted to relatively clean hydrogen in order to obtain good fuel cell performance and long life. Often the initial fuel processing step is steam reforming to produce hydrogen and which is accomplished in a package outside the fuel cell called the reformer. The output gas from the reformer may include on the order of 10% carbon monoxide. This gas may then be further processed to produce additional hydrogen and reduce the carbon monoxide content by using a shift converter. The shift converter output may contain one or two percent carbon monoxide. Even this small amount of carbon monoxide may be unacceptable in a fuel cell where long life is an important criterion. A selective oxidizer is used to further decrease the carbon monoxide content to a tolerable level, on the order of about 0.1% carbon monoxide.
In the selective oxidizer, oxygen, usually in the form of air, is introduced into a catalyst bed along with the fuel being processed. A typical catalyst is platinum supported on aluminum or on carbon. Other noble metal catalysts may be used, but platinum is preferred. The carbon monoxide and oxygen react in the presence of platinum, converting the carbon monoxide to carbon dioxide in a controlled combustion process according to the following reaction: EQU 2CO + O.sub.2 .fwdarw. 2CO.sub.2 + heat (1)
If there is not enough oxygen with which the carbon monoxide can react the CO output from the selective oxidizer will be too high; if too much oxygen is introduced into the selective oxidizer then the hydrogen in the fuel stream will react with the excess oxygen to produce water according to the following formula: EQU 2H.sub.2 + O.sub.2 .fwdarw. 2H.sub.2 O + heat (2)
This latter reaction is undesirable since it reduces the hydrogen content of the fuel stream.
The ideal approach to controlling the selective oxidizer would be to measure the carbon monoxide content in the effluent gas and adjust the input oxygen (air) flow rate proportionally to reduce the carbon monoxide level. This cannot be done since rapid CO detectors (analyzers) are not known. Conventional methods of detecting carbon monoxide, by the use of gas chromatographs, would impose a significant time delay to the process and proper control would not be attained. Further, gas chromatograph equipment is large, expensive, and not suitable for incorporation into a fuel cell power plant.